The device disclosed in Japanese Patent Application Laid-open No. 2001-163233, for example, is a conventional electromagnetic proportional flow rate control valve used for power steering apparatuses.
In the electromagnetic flow rate control valve described above, as shown by dashed lines in FIG. 3, a driving force of a solenoid for driving a valve member is set to have flat characteristics so that the solenoid driving force becomes uniform over nearly an entire region of a valve stroke with respect to an identical driving current.
In general, as shown by double dashed lines in FIG. 3, the solenoid driving force sharply increases on an adsorption side in accordance with a valve stroke with respect to an identical driving current. However, since control characteristics of the valve become complex in this case, the solenoid driving force is set so as to have flat characteristics, thereby being capable of making control of a valve-opening degree easier.
However, even if the solenoid driving force is set to have flat characteristics, between the driving current that flows in a solenoid coil and the solenoid driving force, there is such a relationship that the solenoid driving force becomes relatively large with respect to a change in a unit current value in a region where the current value becomes large. This is because the solenoid driving force is proportional to the square of the current value. The extent of change in the solenoid driving force therefore gradually becomes larger with respect to the same amount of change in the current, as shown in FIG. 3.
As a result, as shown by a dashed line in FIG. 4, the extent of the change in the solenoid driving force with respect to an identical amount of change in current becomes larger in the region where the value of the solenoid driving current is large, compared with a region where the current value is small.
The degree of opening of the valve generally becomes a maximum in a state at which the driving current is zero in the electromagnetic proportional flow rate control valve, and the degree of opening thereof decreases along with the increase in the driving current. That is, the control amount is set so as to decrease in accordance with the increase in the driving current.
The flow rate to be controlled, therefore, had a tendency to change sharply with respect to a slight change in the driving current in the region where the opening degree of the electromagnetic proportional control valve is small. A problem consequently developed in that accurate flow rate control becomes difficult in a low flow rate region where the flow rate to be controlled is small.
An object of this invention is to provide an electromagnetic proportional flow rate control valve capable of performing flow rate control with high precision in the low flow rate control region.